Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight-stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilisation by cross-linking of a polymer, heat-induced solubilisation, decomposition, or particle coagulation of a thermoplastic polymer latex.
EP 0 864 420 discloses a heat mode imaging element for making lithographic printing plates comprising on a lithographic base having a hydrophilic surface an intermediate layer comprising a polymer, soluble in an aqueous alkaline solution and a top layer that is sensitive to IR-radiation wherein said top layer upon exposure to IR-radiation has a decreased or increased capacity for being penetrated and/or solubilised by an aqueous alkaline solution.
EP 0 908 304 and EP 0 908 306 disclose a heat mode imaging element consisting of a lithographic base with a hydrophilic surface and an IR-radiation sensitive top layer, comprising a polymer that is soluble in an aqueous alkaline solution and that is unpenetrable for an alkaline developer containing SiO2 as silicates.
The last two heat-mode imaging elements have the disadvantage that the difference between the solubility in the exposed areas and in the non-exposed areas is not very great so that also non-exposed areas are dissolved during the processing of the element so that the plates could not be used as lithographic plates.
WO 97/39894 describes a positive-working heat-sensitive printing plate precursor which is sensitive to IR light but not to UV light, comprising a support and an IR-sensitive coating comprising an oleophilic polymer that is soluble in an aqueous alkaline developer and a dissolution inhibitor which reduces the solubility of the polymer in the developer.
WO99/21725 and WO99/21715 describe a heat sensitive printing plate precursor provided with a coating comprising a compound which increases the developer resistance of the coating. Said compound is selected from the group of poly(alkylene oxide), siloxanes and esters or amides of polyhydric alcohols.
EP 0 950 517 and EP 0 950 518 describe a heat mode imaging element for providing a lithographic printing plate comprising a base with a hydrophilic surface, a first layer comprising a polymer soluble in an aqueous alkaline solution and an infrared sensitive top layer, wherein at least one of said layers comprise a surfactant such as a polysiloxane.
The prior art printing plate precursors comprising compounds which increase the developer resistance of the coating such as for example polysiloxanes, also have a broad development latitude, i.e. the differentiation between the development kinetics of exposed and non-exposed areas is increased in the sense that exposed areas are completely dissolved before the non-exposed areas start to dissolve. However, the minimum energy density required to solubilize the exposed areas in the developer of these printing plate precursors is high, and therefore, long exposure times and/or the use of more expensive exposure devices such as lasers with a high laser power output are required.
Therefore, there is still a need for printing plate precursors which have an improved sensitivity and at the same time a high under exposure latitude and a high developer resistance.